Treatment of materials by electronic bombardment



Feb. 25, 1958 M. C. CROWLEY-MILLING TREATMENT OF MATERIALS BY ELECTRONIC BOMBARDMENT Filed Jan. 17, 1955 2 Sheets-Sheet 1 TREATMENT OF MATERIALS BY ELECTRONIC BOMBARDMENT Filed Jan. 17, 1955 2 Sheets-Sheet 2 INVENT ATTORNEYS United States Patent 9 TREATMENT OF MATERIALS BY ELECTRONIC BOMBARDMENT Application January 17, 1955, Serial No. 482,287

Claims priority, application Great Britain February 1, 1954 6 Claims. (Cl. 250-495) This invention relates to the irradiation of material by electronic bombardment and has an important application in the sterilisation of foods, chemicals, and medical and surgical apparatus, and the like.

It has been found that high energy electron beams are capable of penetrating substances to a considerable depth and in doing so are effective in destroying living organisms such as bacteria. The sterilisation action of high energy electron beams is extremely rapid and hence articles and materials to be sterilised can be processed in a continuous flow movement across the electron beam.

Examples of apparatus suitable for producing such an electron beam are linear accelerators, the Van de Graaif generator and the betatron.

In applying such apparatus for this purpose it is usual to provide the vacuum enclosure with a window consisting of thin metallic foil hermetically sealed to the surrounding wall of the vacuum chamber, which foil must be strong enough mechanically to withstand the atmospheric pressure but at the same time sufiiciently thin to permit of the passage of the greater part of the electron beam. 7

It will be appreciated, however, that the passage of the beam through such a window will cause a certain amount of scattering depending upon the thickness of the window and the density of the foil material.

Since the original beam is normally of approximately circular cross-section the area irradiated will also be approximately circular. However, in many cases a circular cross-sectional shape is unsuitable, for instance, in cases above instanced in which the material to be irradiated moves continually across the beam, it follows that parts at the centre will cross the full diameter of the beam and hence receive a much larger dose than the parts at the side which will only intersect a sector of the beam. This unequal dosage is accentuated by the fact that the intensity of the beam is not uniform but is greatest at the centre and reduces towards the edges.

The main object of the invention is to provide an improved arrangement for providing a beam, the crosssectional shape of which is elongated so that its dimension along the direction of movement of the materials being treated will be substantially uniform.

According to the present invention an electron beam emitter has means for projecting the electron beam through a magnetic field whereby the beam is deflected laterally so as to traverse a curved path and is also spread laterally in the plane of curvature so that the resultant beam has an elongated cross-section.

Preferably the deflecting field is produced between a pair of fiat faced pole pieces which are splayed outwardly so that their separation is greatest and hence the field weakest towards the outside of the curvature of the electron path.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings, in which:

. 9 underneath Fig. 3.

Fig. 1 shows diagrammatically the' beam normally produced by a high voltage electron accelerator; whilst Fig. 2 is a plan view of a moving belt carrying objects to be irradiated, and indicates the area affected by the beam;

Fig. 3 shows a side view of apparatus embodying the invention; whilst Fig. 4 is a section on the line IV-IV of Fig. 3;

Fig. 5 shows approximately the shape of beam which can be produced with this apparatus;

Fig. 6 shows a modified form of the apparatus shown in Fig. 3.

Fig. 7 shows a design of pole pieces alternative to those shown in Fig. 3 and Fig. 4.

Referring first to Fig. 1, the reference 1 indicates the end of an electron accelerator such as a linear accelerator having a metal foil window 2. The emerging beam is shown as falling on a surface 3 and the propagation of the beam is indicated by the dotted lines 4. The line 5 indicates graphically the distribution of intensity across a section of the beam and it will be observed that the intensity is greatest at the centre of the beam and reduces appreciably towards the edges.

Fig. 2 shows a plan view of the beam of Fig. 1 in which the rays are projected vertically downwards on to the surface 3 which is shown as a carrier for materials being treated, the materials being in the form of reactangular objects 6. The shape of the beam, however, as above explained, will be circular, as indicated by the dotted line 7, and it will be readily appreciated that with a beam of the shape shown in Fig. 2, and intensity distribution as shown in Fig. l, the material along the centre of the conveyor surface 3 will be treated to a much greater extent than that at the outsides of the rectangles.

Figs. 3 and 4 show the improved arrangement embodying the invention. In this drawing, the reference 1 again indicates the outlet end of a linear accelerator having a window 2, whilst the electrons, after leaving the window 2, pass between the pole pieces 8 of a magnet system. These will deflect the beam so that it will follow a curved path, as indicated in the drawing. Whilst the pole pieces are also splayed outwardly, as shown more clearly in Fig. 4, with the result that the magnetic field density is greatest on the inside of the curve and reduces progressively towards the outside of the curve, this will have the effect of elongating the beam in the plane of curvature so that it will ultimately have a shape somewhat as indicated in Fig. 5 and the distribution of intensity will be as indicated by the curve In some cases it may be desirable to insert a second metallic foil 10 in order further to scatter the beam if in elongating the section of the beam in lateral direction too narrow a strip results due to the focusing action of the non-uniform magnetic field in the plane at right angles to that in which the beam is being spread.

In treating materials which are of appreciable thickness it may be found that the electrons only partially penetrate the material and a further feature of the invention consists in splitting the beam into two component beams and applying one component to one side of the material and the other component to the opposite side. A modified arrangement for doing this is shown in Fig. 6 in which the magnet pole pieces 8' are elongated at 11 on the inside of the curve and are so shaped that the beam is split into two component beams 12 and 13, of which the beam 12 bombards the top surface of the mate rial 14. whilst the other half of the beam 13 passes between further magnetic poles, indicated at 15, whereby it is deflected through approximately and bombards the lower surface of the material 14. The field produced Patented Feb. 25, 1958 j i the pole. pieces 8 may either; be pole .piecestoipermanent ,magnets or of'electroamagnets.

pieces 26, as shown in Fig. 7, and so forth. 7

' of curvature, means for splitting-the electron heap;

whereby beemis-cr ssec onn l aa mm hl beam."

'by the poles may be uniform, or, ifjt is desired to;

shape the beam further, non-uniform. 7

.It 'will be appreciated'thatin the arrangement shown Whilst in-the arrangement shown the nonuniform mag- V neti fi' ldi s b n tai edy n sp ay na ut o l pole pieces, giving an approximately-linear variation offield with:displacement fromthe 'mean path it tnay be desirable to have this variation other thanlinear; to achieve any desired variation of intensity across the beam ofelectrons, and this maybe done bytshapingth e pole. pieces, inserting material v0t graded magnetiggrev' s ase .9;.21 1 241 ;;1 e wee a ral l qle Wh mlaim 1. An electron beam emitter. apparatus; ans: r Pr c i n ectm e m aj tasn q 1 producing a magnetic field across said beanf to use e fb m o e w. a curved pat p l n ece .t r a ki- .magnet having a separation which decreases prog resi sively towards the centre of curvature of the beam where by the beams cross-section is elongated in the plane of t re. me s pl n h lec o heam r t ttw component beams separated in the plane o f curg atur e and means for producing a second magnetic fi Id;ac'ross m l pp s x m e y wi V t 2-- PP m$ o s r t x bie s w th. e ec ron p n -me r P ct ane ctr nbe mw ma i net for producing a magneticfield across s aid beam to.

cause said beam to follow acurved path pole pieflces to saidtmagnet having a separation which decreases pro-u gressively 1 towards the; centre of; curvature ,of the beam; 7

whereby the beams cross section is;e1ongate51 in the- 1a e.

two emp t ea s swa n'thelplam icada-1 ture, and means for producing a second magn c field across one of said component beams sp as to;defieg t;it

ou a ang e fipamxim el 8.92 n h tin were? 40 n b e ma i r ate an; p r e s es: by said'two component beams c c ppara s' r rad t n q iect w e ectrons comprising means for projecting an electron. beam a ma ne e vs naa-ma eti fie d. sr ss d een. to e. a d b m. IQ 3 1924 a ,e rredzp th. le pieqe for. said magnet having a, separation which decreases; ss elyww r slheqs'n i of? curvature. and l oiis asidrt ia ie i telu tanse aa serted between said pole piec s itoassist in c intensityofihe e qabeam qy t a rqses s i. n y r to f asart: he; swee hma h ai 4. Apparatus for irradiating: objects with electrons.

comprising'means for projecting an electron beam, a

' magnet for producing a magnetic field across said beam to' cause said beam to follow a curved path, pole pieces for said magnet having a separation which decreases progressively towards the centre of. curvature of the beam 7 whereby! the. beams' fcross-section elongated in the plane of curvature, material of gradd magnetic reluct:

' ands-.inserted. between said: pole. pieces .to,.assist;.in. con-s trolling.the'fintensitvof;the; electron, beam aover its cros s c sectio i,l' neans for splittingfthe electron. beam intomtfwo component ,beams separated in the plane of curvature, 1

V and means for producing a second'fmagneticrfield across} one of st qomaqaent eamin am ddestitfi an angle of'approximately .18Q softhat in operation ob- 1 jects may be irradiated ontwo oppositesides'by said-two component beams ,5. Apparatus for. irradiating objects 'with electrons, comprising means for'projectingian electron beam; a ma 'net forprod c cause said beam" to follow a curvedpath, pole p ieces for' said magnet" having a s'eparation which decreasesproserted betweensaid polep iece'sto' as angle of" approximately I SQ {s othat in operation objects may be adia d n 'vq a ite si i t id 1W0 m-T ponent" beams; and a-conveyor:totransport saidobject"' through said component electronbeams.

6. Apparatus 'fofirr'adiai 'gobject" withelectrons; V c comprisinge'means for projecting" an electron; beam, a

magnet=for produ cinga magnet field across saidbeam tocause said beam to follow a curved path, pole pieces'have 7 ing aseparation which decreases; progressively towards the centre of curvature of the beam whereby the"beams'- cross-'section-is elongated in the plane ofi curvature and a conveyor to transport said objects through said electron beam.

References the; file of this patent N E TATE A ENT I V "Iingley Oct. 14,} 19411 2.602J51 Ro ins n, any 8, 19 .2; 2,680,815 Burril l-- June 8, 1254 

